are you referring to intramolecular hydrogen bonding? in that case intramolecular hydrogen bonding between O- of the carboxylate ion and H atom of -NH2 group present at the ortho position would again make the conjugate base of o-aminobenzoic acid extra stable. which means that o-aminobenzoic acid will be more acidic...
then why is it the least acidic among the 3 compounds?
Which book is this from? I think the only thing the book wanted to point out is the factors that influence pKa which you must learn to account for when you have substitutions in different parts of the ring. These being, as you pointed them out yourself -I effect and the +R effect. The relative acidity of the acid in this case to be honest is a bit of a tough one to determine by theory alone. You are absolutely correct, for the ortho substituted case, after the carboxylic groups deprotonates a strong and favored intramolecular hydrogen bonding takes place between the -NH2 and O-; overall, this favors the deprotonated state of the hydrogen-bond-accepting group (acid moiety), lowering its pKa. However, at the end of the day, at least in this case the -I and +R effects seem slightly more important and a bigger (yet not by much) influence on the pka of the acid.
Here are the Pka values
2-Aminobenzoic acid 4.946
3-Aminobenzoic acid 4.78
4-Animobenzoic acid 4.874
Hope that helps,
Nescafe.